High-temperature Methane Research Articles

Methanol Adsorption on Clean and Oxygen-Predosed V(100) Single-Crystal Surfaces

The thermal chemistry of methanol on V(100) single-crystal surfaces was investigated in ultrahigh vacuum (UHV) by temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and isotope labeling experiments. It was found that thermal activation of adsorbed methanol leads to the production of methane, ethylene, formaldehyde, and carbon monoxide. Methane desorption is seen in two states centered at 320 and 490 K, both rate-limited by the dehydrogenation steps that supply the needed extra hydrogen atoms, from the hydroxyl and methyl moieties, respectively. The presence of submonolayer coverages of oxygen on the surface enhances the yield of the high-temperature methane state at the expense of the low-temperature methane production, and delays all of the surface reactions to higher temperatures. The mechanism for the formation of ethylene in this conversion was investigated by comparing the TPD profiles of several reference molecules. The results suggest adsorbed formaldehyde as the key intermediate and methylene coupling as the most probable pathway.

Characterization of new catalysts based on uranium oxides

Catalysts based on uranium oxides were systematically studied for the first time. Catalysts containing various amounts of uranium oxides (5 and 15%) supported on alumina and mixed Ni-U/Al2O3 catalysts were synthesized. The uranium oxide catalysts were characterized using the thermal desorption of argon, the low-temperature adsorption of nitrogen, X-ray diffraction analysis, and temperature-programmed reduction with hydrogen and CO. The effects of composition, preparation conditions, and thermal treatment on physicochemical properties and catalytic activity in the reactions of methane and butane oxidation, the steam and carbon dioxide reforming of methane, and the partial oxidation of methane were studied. It was found that a catalyst containing 5% U on alumina calcined at 1000°C was most active in the reaction of high-temperature methane oxidation. For the Ni-U/Al2O3 catalysts containing various uranium amounts (from 0 to 30%), the introduction of uranium as a catalyst constituent considerably increased the catalytic activity in methane steam reforming and partial oxidation.

Growth of novel carbon phases by methane infiltration of free-standing single-walled carbon nanotube films

High-temperature methane infiltration of thin, free-standing films of acid-treated single-walled carbon nanotubes (SWCNT) has been studied by means of scanning electron microscopy, transmission electron microscopy and Raman spectroscopy. In the early stages of infiltration, carbon nuclei form predominantly at SWCNT bundle intersections. Further growth proceeds via the formation of graphite nanosheets – without further influence of the nanotube support. Both sheet edges and their structural imperfections act as reaction centers for subsequent deposition, likely giving rise to autocatalytic deposition kinetics. In contrast, infiltration with a H 2:CH 4 (24:1) mixture leads to the reductive activation of residual Ni/Co impurities embedded in the precursor SWCNT-felt. This is associated with a different predominant carbon deposition mode in which multiwalled carbon nanotubes grow out from the substrate.

La-Hexaaluminate Catalyst Preparation and Its Performance for Methane Catalytic Combustion

The La-hexaaluminate catalysts with high performance were synthesized by the modified controllable co-precipitation method with the buffer solution of NH 4HCO 3 and NH 4OH mixture as the precipitation agent. The physicochemical properties of catalysts were characterized by the means of BET, XRD, and TPR techniques. With methane catalytic combustion as the probe reaction, the catalytic performances were also tested on a fixed bed, continual flow system. The results show that it is a good method to obtain chemical homogeneous hexaaluminate materials by the buffer solution as the precipitation agent. The La-hexaaluminate can be formed at low temperatures ranging from 1050 to 1200 °C. The cerium introduction plays a great role in the methane catalytic combustion on La-Mn hexaaluminate because of its high oxygen storage capacity property and the well synergic effect between Ce and Mn. However, the CeO 2 appears in hexaaluminate through the XRD pattern, which reveals that Ce can not enter the crystal lattice position. Mn introduction improves the methane catalytic activity to a large extent due to its high redox property. When Mn atomic substitution amount for Al is 2, the hexaaluminate shows the highest activity, and the catalyst possesses good H 2 consumption and redox performance. Mn can easily occupy the hexaaluminate crystal position, which reveals that the Mn substitute La-hexaaluminate is a promising high temperature methane combustion catalyst with high activity and good stability.

Approximate radiative properties of methane at high temperature

Band model parameters for high-temperature methane have been generated up to 2000 K from an extended spectroscopic database. Part of the spectroscopic data are issued from calculations made in the framework of the effective Hamiltonian approach. These data have been completed by a statistical extrapolation. The calculations of global radiative properties such as band absorbtances and total emissivities are in good agreement with the available experimental data showing that the contribution of the hot bands is correctly taken into account. Finally, the degree of correlation between CH 4, CO 2 and H 2O spectra in typical conditions of combustion applications is discussed. Band model parameters are available upon request.

Time-on stream behavior of Pd-, Co-, and Mn-zeolite catalysts supported on metal blocks in high-temperature methane oxidation

Stability of the Pd-, Co-, and Mn-zeolite catalysts supported on metal blocks was studied in high-temperature methane oxidation. The temperature regions were found in which the starting catalysts exhibit stable performance. The temperature was determined at which a partial deactivation is followed by stabilization of catalysts in reaction environment. In terms of specific activity, the partially deactivated Pd-zeolite catalyst is several times more active than conventional oxidation catalysts Pd/Al2O3, Pt/Al2O3, and the most active oxide CeO·6Al2O3.

High-temperature methane oxidation over metallic monolith-supported zeolite catalysts containing Mn, Co, and Pd ions

The high-temperature complete oxidation of methane over metallic monolith-supported zeolite catalysts containing isolated Mn, Co, and Pd ions was studied. The reaction involves heterogeneous and heterogeneous-homogeneous catalytic processes. The ratio between these processes depends on the temperature, feed rate, and the amount of catalyst charged in the reactor. In the heterogeneous catalytic process, the activity of the catalysts supported on the Fe—Cr—Al monolithic alloy decreases in the series Pd > Mn > Co > Fe—Cr—Al monolith and the reaction rate uniformly increases with increasing contact time. In the heterogeneous-homogeneous process, the reaction rate drastically increases and a 100% conversion of methane to CO2 can be achieved by minor variations of the contact time. In this case, methane oxidation depends not only on the catalyst chemical composition but also on its external surface area and the reaction volume.

Nanoengineered catalysts for high-temperature methane partial oxidation

Novel nanoengineered catalysts for the high-temperature direct oxidation of methane to synthesis gas have been synthesized via a microemulsion-mediated sol–gel route. Through the addition of Pt-salts, catalysts with large surface areas, a well-controlled morphology and very homogeneous distributions of highly active Pt-nanoparticles in a hexaaluminate (BHA) matrix are obtained. The catalysts show syngas yields significantly exceeding the yields over conventional catalysts. Furthermore, excellent high-temperature stability of the materials is observed with no measurable deactivation or metal particle sintering over prolonged reactor operation at high-temperature reaction conditions. Overall, the materials seem highly promising candidates for syngas formation and hydrogen production from hydrocarbons.

Towards simulation of high temperature methane spectra

Methane plays a central role in gas layers of temperatures up to around 3000 K in a number of astrophysical objects ranging from giant planets to brown dwarfs, over proto-solar nebulae, to several classes of cool stars. In order to model and analyse these objects correctly, an accurate and complete list of spectral lines at high temperature is demanded. Predicting high temperature spectra implies, however, predicting hot bands and thus modelling highly excited vibrational states. This is a real challenge in the case of methane. We report the preliminary results of a theoretical study combining the global effective Hamiltonian approach and its computational implementation (STDS package: http://www.u-bourgogne.fr/LPUB/shTDS.html) with semi-quantitative statistical considerations.

An infrared spectroscopic and temperature-programmed desorption study of methyl iodide hydrogenation on Pd( [formula omitted

The adsorption and hydrogenation of methyl iodide is studied on clean and hydrogen-covered Pd(1 1 1) using reflection–adsorption infrared spectroscopy (RAIRS) and temperature-programmed desorption. Molecular methyl iodide desorbs from clean Pd(1 1 1) at 229 K and hydrogen desorbs at ∼360 K. Methyl groups hydrogenate in two states at 191 and 304 K for large methyl iodide exposures where RAIRS data suggest that the low-temperature state is due to reaction with tilted methyl species while the high-temperature state is formed by hydrogenation of perpendicular ones. A single state is found at low methyl iodide coverages at ∼219 K due to the hydrogenation of tilted methyl species where the kinetics is limited by the rate of α-hydrogen elimination. Saturating the surface with hydrogen completely suppresses the high-temperature methane desorption state and the methane desorbs at between 189 and 207 K depending on coverage.

Sulphur poisoning of LaCr0.5−xMnxMg0.5O3·yMgO catalysts for methane combustion

LaCr0.5−xMnxMg0.5O3·yMgO catalysts (x=0, 0.25, y=0, 17) were prepared, characterised (X-ray diffraction, BET analysis, scanning electron microscopy–energy dispersion spectroscopy, transmission electron microscopy, FTIR spectroscopy, chemical and atomic absorption analyses), tested for high-temperature methane combustion and aged in presence of SO2 to investigate the effect of sulphur on their catalytic activity. The role of chromium presence in the perovskite structure have been assessed by comparison with earlier studies on Cr-free perovskites: (i) it worsens the activity of catalysts in the fresh state; (ii) owing to its acidic nature, it makes perovskites less prone to adsorb SO2 and so more resistant to sulphur poisoning; (iii) it renders catalyst regeneration by NH3 leaching less effective because of the partial solubility of chromium oxide in basic media. Conversely, MgO promotes catalytic activity of fresh LaCr0.5Mg0.5O3·yMgO catalysts and slows down sulphur poisoning of the LaCr0.25Mn0.25Mg0.5O3·17MgO catalyst only.

Working mechanism of an ethanol filter for selective high-temperature methane gas sensors

Semiconducting metal-oxide gas sensors are generally nonselective, which limits their use as natural gas detectors in domestic environments when ethanol is present in high background concentrations. Using a thin-film Ga/sub 2/O/sub 3/ sensor with a thick-film catalyst filter of Ga/sub 2/O/sub 3/ and an operating temperature of 800/spl deg/C, the cross-sensitivity to ethanol is strongly reduced and the sensor response to methane is enhanced. Detection of natural gas is made reliable and the rate of false alarms is reduced. Oxidation of ethanol and methane over gallium oxide is studied using GC product analysis. These measurements of catalytic activity help to clarify the reactions involved in the filtering mechanism. Elimination of the ethanol cross-sensitivity is attributed to the thermal combustion of ethanol as it passes over the hot filter. The sensor response to methane is enhanced as methane is activated by the active catalytic Ga/sub 2/O/sub 3/ thick-film.

The Structure of Carbon Encapsulated NiFe Nanoparticles

Carbon encapsulated NiFe nanoparticles (NiFe@C) have been prepared by high-temperature methane encapsulation of the bare bimetallic particles on alumina. High-resolution transmission electron microscopy pictures show that about 6-nm thick carbon layers encapsulate 10–20-nm diameter NiFe nanoparticles. The NiFe nanoparticles are shown to be single-crystalline and no carbide is found at the NiFe–C interface. This is confirmed by the electron energy-loss spectroscopy (EELS) measurements that in addition show that both Ni and Fe have a zero (metal) valence and that only graphite is present. EELS also shows that the nickel-to-iron ratio is exactly unity for all particles studied. Metallic Pd nanoparticles with a diameter of 1–2 nm can be anchored on the carbon layers, which creates a Pd/NiFe@C type of catalyst that could be used for liquid phase reactions. The EELS analysis reveals that part of the nanoparticles present are not Pd but other oxidic carbon encapsulated nanoparticles.

Sulphur poisoning of LaMn1−xMgxO3·yMgO catalysts for methane combustion

LaMn1−xMgxO3·yMgO catalysts (x=0, 0.2, 0.5, y=2, 6, 17) were prepared, characterised (X-ray diffraction (XRD), BET, scanning electron microscopy (SEM)–energy dispersion spectroscopy (EDS), transmission electron microscopy (TEM), FTIR, chemical analysis and atomic absorption), tested for high-temperature methane combustion and aged in presence of SO2 to investigate the effect of sulphur on their catalytic activity. Two important roles of MgO have been assessed: (i) MgO acts as promoter for the catalytic activity of fresh LaMn1−xMgxO3·yMgO catalysts up to a certain MgO excess (y=6); (ii) MgO slows down sulphur poisoning of all LaMn1−xMgxO3·yMgO catalysts. LaMn1−xMgxO3·17MgO catalysts show the highest resistance to sulphur poisoning and their deactivation is appreciable only after 32 days of exposure to 200ppmv SO2 at 800°C. They, then, can be regenerated completely by aqueous NH3 washing. Sulphate formation on both perovskite and the MgO phase is proposed to be responsible of catalyst deactivation. An explanation of the nature of these sulphates, also based on FTIR investigation, is given.

Thick-film coating of hexaaluminate catalyst on ceramic substrates and its catalytic activity for high-temperature methane combustion

Thick-film coating of hexaaluminate catalyst on thermally stable ceramic substrates and the catalytic activity of the film for methane oxidation were investigated for high-temperature combustion applications. The hexaaluminate film coated on pure alumina substrate retained the initial composition after calcination at 1200°C and displayed slightly lower catalytic activity than bulk hexaaluminate catalyst calcined at the same temperature. At the calcination temperatures of 1400 and 1600°C, migration of the components of the film and substrate occurred and the catalytic activity decreased considerably. The hexaaluminate catalyst film heat-treated at 1600°C was almost inactive for methane oxidation, which is attributable to volatilization of Mn during the heat treatment. By direct coating of the hexaaluminate on aluminum titanate honeycomb followed by calcination at 1200°C, the film exfoliated and cracks appeared in the aluminum titanate honeycomb. With an alumina intermediate layer inserted between the film and honeycomb, the thermal stability of the hexaaluminate film on the substrate calcined at 1200°C was significantly improved and the catalytic activity of the film was comparable to that of bulk hexaaluminate catalyst.

A two-step thermochemical conversion of CH4 to CO, H2 and C2-hydrocarbons below 1173 K

A high-temperature thermochemical process using a redox system of metal oxide is proposed for converting CH 4 to CO, H 2 , and C 2 -hydrocarbons (C 2 -HCs). Reactions were performed in a two-step redox cycle. In the first high-temperature and endothermic step, methane is reacted with metal oxide to produce C 2 -HCs and the reduced metal oxide: 2CH 4 +metal oxide→C 2 H n +(4-n/2)H 2 O + reduced metal oxide In the second step, H 2 O or CO 2 is reacted with the reduced metal oxide to generate H 2 or CO at lower temperatures: H 2 O + reduced metal oxide → H 2 + metal oxide CO 2 + reduced metal oxide → CO + metal oxide The net reactions are 2CH 4 → C 2 H n + (4-n/2)H 2 , and 2CH 4 + (4-n/2)CO 2 → C 2 H n + (4-n/2)(CO + H 2 O) A thermodynamic analysis showed that redox systems of Fe 3 O 4 /FeO, SnO 2 /SnO and WO 3 /WO 2 are promising for the two-step process. The redox system of Fe 3 O 4 was experimentally examined. Methane was selectively converted to C 2 -HCs, H 2 , and CO by the two-step process using the SiO 2 -supported Fe 3 O 4 (Fe 3 O 4 /SiO 2 ) in the temperature range from 1123 to 1173 K. It was found that the high-temperature methane decomposition to bulk carbon was efficiently suppressed over Fe 3 O 4 /SiO 2 . This process offers the efficient endothermic net reaction for converting natural gas to C 2 H 4 , H 2 , and CO with upgraded calorific values, utilizing concentrated solar radiation as the energy source of high-temperature process heat below 1173 K.

Application of the mono/multilayer and adsorption potential theories to coal methane adsorption isotherms at elevated temperature and pressure

Accurate estimates of gas-in-place and prediction of gas production from coal reservoirs require reasonable estimates of gas contents. Equations based on pore volume filling/potential theory provide a better fit than the Langmuir equation to both high-pressure (up to 10 MPa), high-temperature (> 1.5Tc) methane isotherm data, and low-pressure (< 0.127 MPa) carbon dioxide isotherm data for 13 Australian coals. The assumption of an energetically homogeneous surface as proposed by Langmuir theory is not true for coal. Application of potential theory to the methane-coal system results in temperature-invariant methane characteristic curves, obtained with the assumption of liquid molar volume of the adsorbate and extrapolated vapour pressures. Temperature-invariant characteristic curves are obtained for carbon dioxide, although further testing is required. The application of isotherms equations based upon pore volume filling/potential theory, in particular the Dubinin-Astakhov equation, have general validity in their application to high-pressure supercritical methane-coal systems as well as providing a better fit to isotherm data.

Growth Rate and Surface Morphology of Diamond Homoepitaxial Films on Misoriented (001) Substrates

Homoepitaxial films were grown on misoriented diamond(001) substrates using microwave plasma-assisted chemical vapor deposition with a methane and hydrogen gas mixture. The dependence of growth rate and surface morphology on methane concentration, substrate temperature and off-angle was investigated. The growth rate dependence was significant for a growth at a high substrate temperature (1000° C) and low methane concentration (1%), suggesting the surface migration distance of the nanometer order. A flat surface was observed macroscopically and microscopically for films grown on off-substrates at a high substrate temperature and low methane concentration.

Kinetic and Thermodynamic Sensitivity Analysis of the NO-Sensitised Oxidation of Methane

Abstract Thermodynamic parameters such as species heats of formation and entropies may have a significant impact on the output of detailed kinetic models, but attention is generally only given to considering kinetic parameters in model development and optimisation. In this paper, a method for comparing the impact of uncertainties in both kinetic and thermodynamic parameters on the predictions of detailed kinetic models is described. This method employs kinetic and thermodynamic sensitivity analysis to define an impact factor for all parameters under consideration as the product of the sensitivity of a particular model output to the parameter and the uncertainty in the value of that parameter. Kinetic and thermodynamic impact factor analysis has been applied to the results of an experimental study of the interaction between NOx (ca. 0-200 ppm)and methane (ca. 40-1300 ppm) in the presence of oxygen (ca. 0-1%) in an atmospheric pressure flow reactor, for temperatures ranging from 500 to 700 C. A detailed chemical kinetic model has been assembled to describe the measured profiles of CH4, CO, C02, NO, and N02. The predictions of the kinetic model are in very good agreement with the experimental data over the entire range of conditions. The NO- and methane-oxidation reactions promote each other, leading to N02 and CO as the main products. The mechanism of methane oxidation was found to be very similar to the high-temperature methane oxidation mechanism, with respect to the intermediate species involved. However, the addition to NO provides a source of chain carriers at unusually low temperatures, since, in the presence of NO, the formation of methylperoxy radicals is no longer chain-terminating: CH302 + NO→CH30 + NO2 dominates the fuel oxidation. This reaction is also the most important NO-oxidising step.

The oxidation of methane at elevated pressures: Experiments and modeling

A detailed chemical kinetic model has been developed for methane oxidation which is applicable over a wide range of operating conditions. A reaction mechanism, originally developed for high-temperature methane oxidation, was expanded and extended to include reactions pertinent to the lower temperature, elevated pressure conditions encountered in the flow reactor experiments performed in the study. The resulting 207-reaction, 40-species mechanism is capable of reproducing the experimental species concentrations for each of the cases studied. The concentration profiles of reactant, intermediate, and product species, including CH 4, CH 2O, CH 3OH, H 2, C 2H 6, C 2H 4, CO, and CO 2, were obtained in the High Pressure Optically Accessible Flow Reactor (HiPOAcFR) facility for temperatures ranging from 930 to 1000 K and pressures of 6 and 10 atm. Based on the model, no appreciable change in reaction pathway was observed over the pressure and temperature range studied, with HO 2 providing the major route for CH 3 oxidation. CH 2O was found to be a vital intermediate for all of the CH 4 oxidation paths. In addition, inclusion of trace amounts of CH 2O measured at the initial sampling location into the model initial conditions greatly reduced the predicted time to onset of fuel disappearance and enhanced the model agreement. This result is consistent with past engine studies which have found that CH 2O is a significant pro-knock additive when added to a methane base fuel. The expanded reaction mechanism was also tested against shock-tube ignition delay and laminar flame speed data and was found to be in good agreement with the relevant experimental data.